The digestive tract generally performs five functions when processing food. The functions include motility, secretory, digestive absorptive and elimination functions. Food processing begins in the oral cavity, which mechanically breaks down food by mastication, lubrication with saliva, and enzymatic processing. Food processing continues in the stomach where food is liquefied by gastric juices and enzymes secreted by the cells lining the stomach to produce chyme. The chyme next enters the small intestine where bile salts produced by the liver and digestive enzymes produced by the pancreas further process the chyme. The small intestine also absorbs components from the chyme through its walls. Components that are not absorbed by the small intestine enter the large intestine. Finally, the large intestine propels waste products into the colon, where they remain, usually for a day or two, until the feces are expelled by a bowel movement.
Sometimes, a person takes an abnormally long time to process food, or a person processes food abnormally fast. Such abnormal gastrointestinal functions are commonly caused by a disorder in the stomach. A stomach disorder can cause the stomach to empty food into the small intestine too quickly or after too long of a time. Stomach emptying disorders can be diagnosed by measuring the rate at which a meal empties from the stomach and enters the small intestine (the “gastric emptying rate”). When the rate is accelerated, the stomach prematurely dumps food into the small intestine, giving rise to abnormally fast rates of gastric emptying (rapid or accelerated gastric emptying). Conversely, when the rate is delayed, the stomach delays moving food into the small intestine, giving rise to abnormally slow rates of gastric emptying (delayed gastric emptying). Hence, measurements of gastric emptying rates have important clinical utility.
Various methods are known for quantitatively measuring gastric emptying rates. One known method is scintigraphy, which has historically been considered the gold standard method of measuring gastric emptying rates. In a scintigraphy method, a subject ingests a meal including at least one edible food, a component of which has been radiolabeled with a gamma emitting radionuclide. A scintillation camera or gamma counter scans the stomach at different time points over an extended time period. The scans directly measure the gamma emissions arising from the radiolabeled meal remaining in the stomach.
Two metrics are commonly utilized in scintigraphy methods. These are (1) fraction of test meal emptied at a number of different time points and (2) the time it takes to empty 50% of a standard test meal from the stomach, or “t ½.” A scintigraphic t ½ value for a test subject may be obtained from using the fraction emptied values by linear interpolation between the two time points at which scintigraphic fraction emptied values bracket 0.5 (50% emptied). Thus, scintigraphy methods use metrics that require taking measurements at a number of different time points.
Another method for measuring gastric emptying is breath testing. In a breath testing method, a subject ingests a meal including at least one edible food, a component that includes a 13C label. As the subject's digestive tract processes the 13C labeled food, a labeled digestive metabolite, 13CO2, is produced which can be detected in the subject's breath. The 13C label passes through the stomach, is absorbed by the small intestine and is metabolized by the liver to give rise to 13CO2. The 13CO2 then moves through the blood to the lungs and exits the body through the subject's breath. The rate of excretion of 13CO2 in the breath signals the rate at which the stomach is emptying. During breath testing, a test administrator collects breath samples from a subject at a number of different time points. Thus, breath testing also requires taking measurements at a number of different time points.
Both scintigraphy and breath testing methods are multi-point tests. Following consumption of a meal including the label, successive measurements are obtained at multiple time points, for example at time points at selected intervals for up to six hours. Such test methods therefore are inconvenient because they require taking measurements at multiple time points and require a long period of time to complete.